What's a good rule of thumb here? My 390 has 9.6:1 stock ratio and should be very close to that after machining, can I run the cheap gas with this? If it makes any difference, it'll be in a 4300# thunderbird, with tri-power, and a Comp 268H (I beleive). If I can't use the cheap stuff and since I have to get new pistons anyways I might as well use the tri-power 10:1 pistons. Thanks, Eric
Flexi Thats the same comp ratio as my Galaxie. I run mid grade when its cold and good gas when the temps get up in the 90s. I've run it on low grade in the winter without any problems but it runs better on the higher octain gas.
If i where you i would use the expensive stuff and stick the 10:1 pistons in. At least you will get better protection against pre ignition and detonation and you can advance the ignition slightly to get that tiny bit more HP that we all want.
My compression is 10.2:1 and I can't run on the cheap stuff. Middle grade (89) seems to work fine, but on a hot day sometimes it'll start to diesel as I turn it off. With 10:1 compression, you should run premium
Pretty much the same as everyone said.. here's a few more examples. my vette with stock 350 and 11.0:1 won't run on anything but premium. My truck with '71 460 10.5:1 will run on med-grade with the timing set back a little. I think if everything else is in good shape on your engine you will be able to run the cheap stuff if you are careful and keep the timing reasonable.
hhmmm... my dilemna is if I can run on the cheap stuff with stock compression and not have to screw with timing that would be ideal. But if I am doomed to the expensive stuff I should at least take advantage of all that octane right? I'm leaning towards the 10:1 pistons...
I've always heard that 10.5 is about the accepted limit for pump premium. Isn't the cam profile also somewhat of a factor? (affects cylinder pressure). Dave
That kind of confuses me, my Mom's new Acura ('03 TL Type S) has 11:1 comp. and runs on the cheap stuff. of course with EFI they can control everything to prevent detonation, BUT when you get on the gas it goes like a raped ape. So it must advance the timing and give it more gas, so why doesn't it knock then?
[ QUOTE ] That kind of confuses me, my Mom's new Acura ('03 TL Type S) has 11:1 comp. and runs on the cheap stuff. of course with EFI they can control everything to prevent detonation, BUT when you get on the gas it goes like a raped ape. So it must advance the timing and give it more gas, so why doesn't it knock then? [/ QUOTE ] You pretty much answered it -- computer controls have a lot to do with it. I think it's also important to consider the fact that your Acura's heads, pistons, etc. were designed in an age when low-octane, no-lead gas was the norm. They're optimized for that sort of fuel. Bottom line is that engineers have simply learned a hell of a lot more since they designed the various V8s a half century (or more) ago. And with computers, they have a lot more capability to optimize ignition advance, fuel delivery etc. That's why we now have V6 minivans that (sadly) make as much power as a strong, hot rodded flathead! Dave
I assume You have iron heads. I would stick to the lower compresion and still run medium grade at minimum. FEs are prone to detonation. Seems like all the ones Ive seen come through My buddies machine shop have lage detonation windows in the bores. The top ring is close to the top of the piston whitch aids to the problem. -- FEDER
It's all about effective compression ratio - or - as I've seen it called lately, dynamic compression ratio. The 9.6/1 CR you listed is a calculated figure - and a usable one. The cam you're running should bleed off some of the compression and I'd say you could probably get by with 87 octane and the stock timing figures. Although I'd bring the all-in centrifugal timing point down to about 2600-2800 rpm for best performance. And limit the vacuum advance to the max vac adv listed in the manuals. Then, at idle you'd have what the factory says your engine likes. If you're just cruising with an occasional burst of full throttle, make sure the centrifugal and vacuum advance mechanisms are working like they should and leave em all stock. Fwiw - I run 9/1 CR and a cam much smaller than yours. With 87 octane there's no pinging or rattling regardless of weather and the engine doesn't diesel or run-on when shut off.
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